1. Field of the Invention
This invention relates to a rubber composition containing rubber composed mainly of ethylene, propylene and diene and having improved weather resistance, thermal resistance, resistance to crack growth and wear resistance. Further, it relates to pneumatic tires, particularly large size tires having considerably improved appearance and retreading life by using the above rubber composition in a tire sidewall as well as pneumatic tires having considerably improved appearance and resistance to groove cracking by using the above rubber composition in a tire tread.
2. Related Art Statement
Heretofore, three has been developed a method wherein rubber composed mainly of ethylene and propylene is copolymerized with a third ingredient such as dicyclopentadiene, ethylidene norbornene, 1,4-hexadiene or the like for curing the rubber with sulfur. The resulting terpolymer rubber (EPDM) has improved weather resistance and thermal resistance because there is no cutting of the main chain due to chemical attack of ozone or the like owing to the absence of double bonds in the main chain.
However, the resistance to crack growth and wear resistance are fairly inferior as compared with those of diene rubber. Further, the terpolymer rubber is poor in the covulcanizability with diene rubber and the application for rubber articles is considerably restricted.
In the field of pneumatic tires, the rubber composition for use in a tire sidewall has frequently been composed of a blend of natural rubber, butadiene rubber (BR) and styrene-butadiene copolymer rubber (SBR). However, the use of BR having an excellent bending durability is increasing as a recent trend. Furthermore, the blend of natural rubber and BR is dominant over the world, wherein the blending ratio of BR becomes not less than 50%. Since the rubber composition composed of only the above diene rubber is easily apt to be oxidized by oxygen, ozone or the like, it is usually compounded with a strong amine series antioxidant and a paraffinic wax for preventing the oxidation of the rubber composition. In new tires made from the compounded rubber composition, there is no problem as to the weather resistance, but the effective amount of the antioxidant in the rubber composition is decreased at the last use stage or in the long-period use based on retreading to cause ozone cracks and the like. The latter problem is highlighted as the tire performances are considerably improved.
As countermeasures for solving this problem, there are considered a method of increasing the amount of amine series antioxidant, a method of using an antioxidant having a long life, a method of using a reactive antioxidant capable of reacting with a main chain of rubber, and so on. However, increasing the amount of amine series antioxidant results in only slightly improved durability Further, the use of a long-life antioxidant or reactive antioxidant only slightly improves age resistance.
On the other hand, it has been attempted to use rubber containing no or a little diene ingredient and having substantially no double bond in its main chain (hereinafter referred to as non-diene rubber) so as not to use the antioxidant. Such a non-diene rubber includes ethylene-propylene-diene terpolymer rubber (EPDM), halogenated butyl rubber, chloroprene rubber and the like, but is rarely practised at the present because the bending durability and resistance to crack growth are considerably poor as compared with those of the diene rubber.
Lately, it is strongly demanded to improve the appearance of the tire itself in addition to the aforementioned problem. That is, when the rubber composition for the tire sidewall contains large amounts of amine series antioxidant and paraffinic wax as previously mentioned, the antioxidant and wax are bloomed on the rubber surface to form a protective layer developing an excellent age resistant effect. However, when the protective layer is exposed to ultraviolet rays, the amine series antioxidant is discolored brown to considerably degrade the tire appearance. Particularly, in order to prevent the above phenomenon in tires for passenger cars, various studies have been made from viewpoints of the improvement of antioxidant and the use of non-diene rubber, but there has not yet been found an effective prevention method.
Furthermore, the rubber composition for use in a tire tread has frequently been composed of a blend of natural rubber, BR and SBR. In case of large size tires, since the ground contact pressure per unit area at the tread surface is large and also the heat release value of rubber is large, the tire tread is generally constructed with a rubber composition composed mainly of natural rubber having a small heat evolution and a large breaking strength. On the other hand, in the tires for passenger cars, the pressure at the tread surface and the heat release value are small as compared with those of the large size tire, and also the heat build-up is less because of the thin tread gauge. Therefore, in order to provide high traction, particularly high friction coefficient at wet state, the rubber composition for the tread of the passenger car tire is composed mainly of SBR.
In the tire tread, the gripping property (particularly at wet state) and wear resistance are important properties, which have been improved by various methods up to date. However, weather resistance and resistance to crack growth, which were insignificant in the past, cause a problem with the improvement of wear resistance. This problem is a phenomenon of producing cracks in the groove bottom of the tread (i.e. groove crack). Such a phenomenon is observed in the tire, particularly after long use, when a greater part of the amine series antioxidant originally contained in the rubber composition has been consumed; this particularly becomes a serious problem at a high temperature area consuming a large amount of the antioxidant.
Many studies have been made in order to solve this problem. For instance, there have been attempted a method of increasing the amount of amine series antioxidant used, a method of using non-diene rubber composition in a tread, a method wherein a thin rubber sheet having good weather resistance is placed on the tread surface (because the surface portion of the tread is lost by wearing, while the groove portion thereof still remains at the last running stage) and so on. Among them, the increase of amine series antioxidant seems to be an immediately effective method. However, it is very effective for the weather resistance at initial running stage, but the effect of improving the weather resistance at last running stage is very small. This is due to the fact that since the amine series antioxidant improving the weather resistance is rapidly bloomed on the tread rubber surface to form the protective layer as previously mentioned, even when the amount of the antioxidant added is made large, the thickness of the protective layer is merely thickened and the holding time itself can not be prolonged.
As to the use of non-highly unsaturated rubber composition, the development is backward because the conventional non-diene rubber (e.g. butyl rubber, halogenated butyl rubber, conventional EPDM, chlorosulfonated polyethylene rubber or the like) is poor in wear resistance and is used with difficulty in a tread rubber composition requiring weather resistance as an important factor.
In the case of placing the thin rubber sheet on the tread surface, the use of non-diene rubber containing no double bond in its main chain is effective as the thin rubber sheet having a good weather resistance, but the adhesion to diene rubber (covulcanizability) is poor, so that there is a problem that even if the rubber sheet is apparently adhered to diene rubber during a static test, peeling is caused therebetween due to dynamic deformation suffered during the running of the tire. Consequently, the use of the thin rubber sheet is hardly practised.